1. Technical Field
This invention is related to the packaging of semiconductor dies in general and, in particular, to a method and apparatus for connecting the die of a high power semiconductor device to an associated substrate with a metal strap that is exposed through a protective body of material encapsulating the entire assembly, thus providing for low electrical resistance and high thermal dissipation.
2. Related Art
FIGS. 1 and 2 are top plan and side elevation views, respectively, of a prior art embodiment of an 8-lead standard outline integrated circuit (xe2x80x9cSOIC-8xe2x80x9d) lead frame-type of power MOSFET semiconductor device 10 that has a conductive metal strap 12 electrically connecting a semiconductor die 14 to a metal portion 16B of a lead-frame-type substrate 16.
In the exemplary prior art MOSFET device 10 of FIGS. 1 and 2, the drain terminals of one or more individual MOSFETs (not visualized) formed in the die 14 are electrically connected to a metal or polysilicon first conductive layer 23 located on a downwardly-oriented first surface of the die. First conductive layer 23 of is, in turn, electrically connected to the metal die paddle 20 of a first portion 16A of the lead frame 16 by, e.g., a layer 22 of solder or an electrically conductive adhesive. The die paddle 20 is integrally connected to each of four leads (5-8) of the first portion 16A in this embodiment.
The source terminals of the one or more individual MOSFETs in the die 14 are connected to a metal or polysilicon second conductive layer 24 (FIG. 2) on an opposing upwardly oriented second surface of the die. The second conductive layer 24 on the die 14 is electrically connected to a second portion 16B of substrate 16 through an electrically conductive layer 36, metal strap 12, and electrically conductive layer 38. Second portion 16B includes leads 1-3 in this embodiment.
The strap 12 comprises a planar cover portion 30 at an inner first end thereof, a planar flange portion 34 at an opposite outer second end thereof, and an oblique connection portion 32 between cover portion 30 and flange portion 34. Cover portion 30 includes a first surface 30a that is electrically connected to the second conductive layer 24 by electrically conductive layer 36, and an opposite second surface 30b that faces toward the top of the package when oriented as shown in FIG. 2. Second surface 30b and all other portions of strap 12 are covered by encapsulant 18. The flange portion 34 of the strap 12 connects to the second portion 16B of the substrate 16 by a lap joint using an electrically conductive layer 38.
The metal strap 12 is made of a conductive metal, typically copper or a copper alloy. As illustrated in FIG. 2, with the device 10 oriented as shown, the first surface 30a of the cover 30 and bottom surface of flange 34 portions of the strap 12 lap over the top surfaces of the die 14 and the second portion 16B of the substrate 16, respectively, and are respectively joined thereto with layers 36, 38 of, e.g., solder or an electrically conductive epoxy.
The gate terminals of the individual MOSFETs are connected to a third conductive layer, or gate pad 26, located on the upper second surface of the die 14 (i.e., the same surface of die 14 as second conductive layer 24). The gate pad 26, which is electrically isolated from the second conductive layer 24, is electrically connected to a third portion 16C of substrate 16 by a bond wire 28 (see FIG. 1). Third portion 16C includes a lead 4, and is electrically isolated from, but coplanar with, first and second portions 16A and 16B of substrate 16.
Encapsulant 18 fully encloses die 14 and metal strap 12, and encloses a portion of the lead frame substrate 16, leaving the ends of leads 1-8 exposed for connection to other circuit components. Encapsulant 18 is typically molded plastic.
Pertinent dimensions of device 10 include: an area of die 14 that is 2 by 2 to 8 by 8 mmxc3x97mm; a height of die 14 that is 0.2 to 0.3 mm; a height of cover portion 30 that is 0.1 to 0.2 mm; and a thickness of encapsulant 18 above second surface 30b that is 0.1 to 0.3 mm. The width of strap 12 is primarily a function of the area of die 14 and second conductive layer 24. The length of strap 12 is a function of the area of die 14 and second conductive layer 24 and the closeness of paddle 20 to second portion 16B of substrate 16.
Metal strap 12 of device 10 provides for improved thermal dissipation characteristics and lower electrical resistance in comparison to earlier configurations in which bond wires were used rather than metal strap 12. Despite the improvements offered by the use of metal strap 12, however, a need exists for a method and apparatus that provides an even greater degree of heat dissipation.
The present invention provides methods and apparatuses for packaging a semiconductor die. Among other features, the packages provide increased heat dissipation capabilities compared to prior art designs.
In one embodiment of a semiconductor device within the present invention, a high power semiconductor die is electrically connected to a substrate on which the die is mounted, e.g., a lead frame, with a metal strap. The die, substrate, and metal strap are encapsulated by an insulative, protective body in such a way that a portion of the metal strap is exposed to the outside environment. The exposed surface allows for increased thermal dissipation and less heat buildup during operation of the device.
An exemplary method for making such a package includes the provision of a semiconductor die, an interconnective substrate, and a conductive metal strap. After the die, substrate, and metal strap are electrically interconnected, an insulative encapsulant is provided (e.g., by molding) over the die and portions of the substrate and strap in a manner that leaves an upper surface of the strap above the die exposed to the environment. In other embodiments, recesses may be formed in the exposed surface of the strap to further increase heat dissipation. In a further alternative embodiment, heat-radiating metal protrusions may be formed on, or fastened, to the exposed surface of the metal strap.
The present invention is also applicable to other types of semiconductor devices. In one embodiment, a first surface of a metal strap is thermally connected, but not electrically connected, to an active surface of a memory, logic, or microprocessor die within rows of edge bond pads formed on the active surface of the die. An opposite surface of the strap is exposed to ambient through the encapsulant, which results in improved thermal dissipation.
These and other aspects of the present invention may be better understood in view of the accompanying drawings and the following detailed description.